Although there have been no examples of commercially produced inductively coupled plasma MS/MS mass analyzers (ICP-MS/MS) up to now, there have been many examples that have been constructed and used in experimental research. An ICP-MS/MS is made up of an inductively coupled plasma (ICP) ion source and an MS/MS mass analyzer (MS/MS) connected to it. The inductively coupled plasma ion source produces plasma containing the sample to be analyzed. The MS/MS mass analyzer is constructed from an interface and an ion lens set, a collision/reaction cell, two mass filters respectively provided on the front end and back end sandwiching the cell, and a detector such as an electron multiplier. The two mass filters are means of separating and extracting ions. For example, they separate certain ions in the ion beam according to mass-to-charge ratio using a quadrupole mass filter. The collision/reaction cell introduces a reaction gas having a relatively low molecular weight such as hydrogen, and by collision and reaction of the reaction gas molecules with polyatomic molecule ions in the ion beam introduced from the front-end mass filter, it selectively neutralizes them and prevents interference with the measurement signal.
By such a configuration, the plasma produced by the inductively coupled plasma (ICP) ion source is introduced into the mass analyzer (MS/MS) as an ion beam via the interface, and ions of a prescribed mass-to-charge ratio are separated by the front-end mass filter and sent to the collision/reaction cell. There is the possibility of the ion beam output by the front-end mass filter containing multiple species of ions having the same mass-to-charge ratio. This ion beam collides and reacts with the reaction gas in the cell, and polyatomic molecule ions having a smaller or larger mass-to-charge ratio are produced, and are sent to the back-end mass filter. The back-end mass filter further separates the ions that are the target of measurement according to a prescribed mass-to-charge ratio, and sends them to the detector.
Thus, the ICP-MS/MS is an instrument which efficiently separates measurement target ions from interfering ions using two mass filters and a cell, and quantifies them. In “Some Current Perspectives on ICP-MS,” D. J. Douglas, Canadian Journal of Spectroscopy, Volume 34, No. 2, 1989 (“Non-patent Reference 1”), the content of which is incorporated herein by reference in its entirety, the article introduces an experiment which demonstrates that ions input to the detector can be selectively reduced in number by utilizing an ion molecule reaction in an ICP-MS/MS. That is to say, terbium ions (Tb+, mass number 159), cerium ions (Ce+, mass number 140, 142) and cerium oxide ions (CeO+, mass number 156, 158) are sent from the ion source through the front-end mass filter and are introduced into the collision/reaction cell which uses oxygen (O2) as a reaction gas. In the cell, Tb+ and Ce+ react with O2 to form TbO+ (mass number 175) and CeO+ (mass number 156, 158), which are sent to the back-end mass filter. As a result, by operating the back-end mass filter at a mass-to-charge ratio that is 16 higher than the front-end mass filter, terbium and cerium can be respectively detected at a mass number that reacts to TbO+ and CeO+. On the other hand, since almost no CeO2+ (mass number 172, 174) is formed in the cell, cerium based oxide ions are limited to CeO+. As a result, almost no CeO2+ of mass number 172 and 174 pass through the back-end filter. That is, the CeO+ signal can be dramatically reduced with respect to the Tb+ signal by utilizing the difference in ion molecule reactions in the cell. As demonstrated by this experiment, ions can be selectively reduced in number using ion molecule reactions in an ICP-MS/MS, and thus, based on this principle, an ICP-MS/MS can reduce the number of interfering ions with respect to measurement target ions.
An ICP-MS/MS must maintain vacuum inside the analysis chamber. International Publication No. WO 00/16375 (Japanese Unexamined Translation of PCT Application 2002-525801) (“Patent Reference 1”), the content of which is incorporated herein by reference in its entirety, is an example which illustrates a pumping configuration for doing so.
As described above, in an ICP-MS/MS, two vacuum chambers in which quadrupoles are arranged are provided before and after the vacuum chamber that holds the cell into which reaction gas is supplied. For reference, FIG. 2 of Patent Reference 1 is appended here as FIG. 6. Patent Reference 1 relates to a vacuum system of an ICP-MS/MS, and discloses that a vacuum chamber which holds a conventional extraction electrode and a collision/reaction cell is divided into a first vacuum chamber 6 which has an extraction electrode and a quadrupole, and a second vacuum chamber 20 which has a collision/reaction cell, and the pressure of the first vacuum chamber 6 is from about 1×10−2 Pa to 1 Pa, typically about 1 to 2×10−1 Pa, and the pressure of the second vacuum chamber 20 is about 1 to 2×10−2 Pa. A third vacuum chamber 33 is provided on the back end of the second vacuum chamber 20 and has a quadrupole mass filter 37 and a detector 38, and pressure of the third vacuum chamber is about 1×10−4 Pa. The first vacuum chamber 6 is made up of a region 14 in which the extraction electrode 8 is housed and a region 15 in which the quadrupole 17 is housed. These regions are vacuum-pumped by a turbomolecular pump as one vacuum stage. The vacuum chambers are connected to each other by orifices 19 and 32 having a diameter of about 2 to 3 mm, but these regions 14 and 15 are connected by an orifice 11 which has a relatively large diameter of about 20 mm so that they have the same pressure. However, such a pumping configuration is a problem according to the findings of the present inventors.
That is to say, as described above, the vacuum pumping configuration proposed in Patent Reference 1 is designed such that the pressure of the first vacuum chamber 6 is about 1 to 2×10−1 Pa. Further, in actuality, the first vacuum chamber 6 is a second vacuum stage which is differentially vacuum-pumped from the ion source 1 which is at atmospheric pressure, and its pressure can only be reduced to about 1×10−2 Pa at best. Incidentally, since the potential applied to the quadrupole 17 consists of high-voltage alternating current of relatively high frequency, normally several MHz or several kV, superimposed over direct current of several hundred volts, there is risk of background noise being high when an ICP-MS/MS is operated at such voltage. Further, to ensure that the quadrupole has sufficient mass selectivity and mass resolution, ion flight distance of about the same length as the quadrupole must be provided. However, the mean free path of ions depends on the ion species. For example, assuming collisions between Ar ions and Ar gas molecules, it is shorter than 30 cm under such pressure condition. For this reason, mass selectivity and mass resolution may be insufficient, and there is also that concern that sensitivity decreases due to collisions between ions and gas molecules. Conversely, if the length of the quadrupole is shortened in order to prevent a decrease in sensitivity, the mass resolution of the quadrupole itself is sacrificed, and there is the problem that analysis performance of the ICP-MS/MS is reduced due to an increase in spectral interference.
Therefore, there is a need for further improvements in vacuum pumping configurations utilized in ICP-MS/MS systems.